This invention relates to an electromechanical transducer device. More particularly, this invention relates to high power ultrasonic transducers.
High power ultrasonic transducers have been utilized for many years in applications such as thermoplastic welding, biological processing, degassing of fluids, ceramic milling and localized cleaning. Examples of current art are those manufactured by Heat Systems, Inc. of Farmingdale, N.Y., and Branson Sonic Power Corp. of Danbury, Conn.
These transducers are constructed in the style known as a Langevin sandwich, wherein one or more piezoelectric crystals and a corresponding number of thin metal electrodes are fitted between two masses of acoustically efficient metals, such as aluminum or titanium, and held in a stressed condition by a center bolt. Typical embodiments of this construction are described in U.S. Pat. Nos. 3,328,610, 3,368,085 and 3,524,085.
When a sinusoidal electrical signal is applied across the polarized crystals via the thin metal electrodes, the crystals begin to vibrate, due to the inherent nature of piezoelectric (a/k/a electrostrictive) materials. This phenomenon is well known to those schooled in the art. By shaping the front and rear masses properly, the natural frequency of resonance of the total stack may be adjusted separately from that of the individual crystal elements and the stack becomes an efficient motor for driving a variety of tuned elements, known as horns. These may be simple cylinders, or complex cylindrical or rectangular shapes suited for welding such thermoplastic items as automotive tail-light lenses, medical filter housings and toys.
When the horn is to be a solid shape and used for applications such as the ones listed above, the transducer stack is efficient and suitable. However, a host of applications exist where it is desirable to introduce liquid and/or gas to the working surface of the horn tip or to aspirate fluid or gas from the area surrounding the tip via suction. Examples of these applications are the atomization of liquid, surgical devices for tumor/tissue removal and liquid processing such as homogenization of dissimilar or immissible fluids.
An examination of prior art reveals a plethora of designs seeking to accomodate fluid pathway to the tip (distal) end of the tooling. Examples of such designs may be found in U.S. Pat. Nos. 3,464,102, 4,153,201, 4,301,968, 4,337,896, 4,352,459, 4,541,564 and 4,886,491.
Generally, these designs seek to introduce liquid into the transducer at a nodal point or through the center of the transducer via an axial hole. Another solution to the problem of introducing fluids to or removing fluids from a distal end of an ultrasonic device seeks to introduce the liquid at the nodal point of the horn itself. An example of this type of unit is the Model 434 FLO-THRU horn, manufactured by Heat Systems Inc. of Farmingdale, N.Y.
Introducing the liquid (or aspirating the fluid) from the node point of either the transducer or the horn has proven to be adequate if the liquid or gas is free from significant amounts of solids, has a viscosity not significantly greater than that of water and does not solidify readily. However, if any of these conditions exists, the design is prone to clogging or cross contamination of the fluids from batch to batch, since cleaning of passageways is difficult, at best. The fluid pressure needed to overcome the right angle bend within the device is also greater than if the fluid path was straight. This greater pressure yields more loading on the stack, thereby reducing the electrical efficiency of the system.
A more important drawback becomes apparent upon a review the theory of the motion of a body subjected to standing wave vibrations. As is well known in the art, a bar of material with both ends free and subjected to either transverse or longitudinal vibrations has imposed upon it locations of relatively high particle displacement and locations of low or nil particle displacement. These locations are known respectively as anti-nodes and nodes.
Any material which comes in contact with the areas of high displacement are prone to be coupled to the ultrasonic vibration of the bar. This, in fact, is the theory of operation of an ultrasonic welder, wherein the thermoplastic or thin metal is acoustically vibrated to raise the internal temperature of the material to allow welding. It is accordingly clear that liquid connections, mounting hardware, etc. should only occur at places of no movement, i.e., node points.
However, it is to be noted that node points are theoretical single points along the length of the crystal stack. Practically, it is difficult, if not impossible, to mount a liquid fitting of any size to this node point without it becoming part of the vibratory load. For this reason, the fittings are generally connected to flexible tubing, so as not to vibrate the fittings loose, or worse still, cause fatigue failure of the tubing material.
In addition to the size of the connections, another drawback of this type of construction is that the location of the node point will change as the stack heats or is loaded. This fact exacerbates the problem of mounting the protective case to the stack as well, since an improper mounting location will cause the case to vibrate.
A design improvement currently known in the art moves the liquid entering point to the rear of the unit and allows an axial path through the transducer. With this construction, the path is straight, which allows cleaning with a variety of mechanical brushes, rods, etc. In addition, the straight path imposes the lowest pressure requirement for the liquid stream, easing the design of the pumping system. Since the liquid connection is at the back of the transducer case, the liquid connection may be made concentric with the axial centerline, which lowers the overall dimension of the device and allows a more ergonomically correct system when used in surgical applications.
Although the design offers these improvements, it presents a practical problem for the design of a device which is both functionally suitable as well as manufacturable. Some limitations of the design can be described as follows.
In order to incorporate an axial pathway, the center bolt must be hollow. This immediately presents the problem of how to seal the threads against fluid seepage, since any liquid which enters the crystal stack will lead to electrical shorting or liquid cavitation in the vicinity of the crystals themselves, which serves to heat the stack to high temperatures very rapidly. Both phenomena will lead very quickly to transducer failure.
In order to solve this problem, designers will generally incorporate an O-ring type of seal or seek to seal the threads with a commercially available thread sealant. Both of these solutions are stopgap, since they are prone to failure with time, as the elastomers or sealant lose their compliance.
Another practical limitation of this design is the attachment of the bolt to the end plate of the transducer. As can be appreciated by those schooled in the art, the center bolt, the liquid connection and the rear cover of the transducer case should be one piece in order to be liquid tight. If this design is to be functional, the stack will be designed so that the entire stack enters the case from the rear, with the stack being supported by the solid liquid tube. Although this allows assembly of the system, the case cover and the case are now part of the vibratory load, since the center bolt is now part of the liquid pathway. As has already been discussed, the loading of vibratory elements with static elements should be avoided, since it tends to detune the stack (changes its resonant frequency) and can lead to heating and rapid destruction of the transducer.